I used to wonder about that too, but after all if Lorien could bring him back from the dead, why not have a good haircut to go with it?

The other thing that interested me is in the introduction to Season 4, Sheridan is shown with his helmet on, and I was always convinced that his hair was going to change during the season. Sure enough, by the end of season 4 he had grown a beard.

It seems his real wife, Melissa, hated the beard and by season 5 he had removed 90% of it and kept just a miniscule amount of moustache and beard. Apparently JMS wanted a beard so he had to keep it but he got away with the least possible.